1. Field of the Invention
The present invention relates to a propylene-ethylene block copolymer and to a molded article made of the same. More particularly, the invention relates to a propylene-ethylene block copolymer which is superior in stiffness, hardness and formability and also in balance between toughness and low-temperature impact resistance when being fabricated into a molded article and to a molded article made of the same.
2. Description of the Related Art
Polypropylene resin compositions are materials superior in stiffness, impact resistance and the like and are in use for wide range of applications in the form of molded articles, e.g. automotive interior and exterior parts and housings of electric components. It is well known that, among such polypropylene resin compositions, those comprising propylene-ethylene block copolymers, for example, polypropylene resin compositions comprising a propylene-ethylene block copolymer and a propylene homopolymer or a polypropylene resin composition comprising two or more kinds of different propylene-ethylene block copolymers are suitably used due to their superior stiffness, impact resistance and the like.
For example, JP-A-7-157626 discloses a thermoplastic resin composition comprising a propylene-ethylene block copolymer obtainable by multi-stage polymerization and a polyolefin rubber. As the propylene-ethylene block copolymer, used is one comprising a propylene ethylene block copolymer which contains a propylene-ethylene random copolymer phase having an ethylene content of from 5 to 50% by weight and also having an intrinsic viscosity of from 4.0 to 8.0 dl/g and a propylene-ethylene block copolymer having an ethylene content of more than 50% by weight but not more than 98% by weight and also having an intrinsic viscosity of not less than 2.0 dl/g but less than 4.0 dl/g. The publication also discloses that a thermoplastic resin composition with an extremely great ductility is obtained.
JP-A-7-157627 discloses a thermoplastic resin composition comprising a propylene-ethylene block copolymer obtainable by multi-stage polymerization and a polyolefin rubber. As the propylene-ethylene block copolymer, used is one comprising a propylene-ethylene block copolymer which contains a propylene-ethylene random copolymer phase having an intrinsic viscosity of from 4.0 to 8.0 dl/g and a propylene-ethylene block copolymer having an intrinsic viscosity of not less than 2.0 dl/g but less than 4.0 dl/g, with the proviso that that a propylene-ethylene block copolymer which contains a propylene-ethylene random copolymer phase having an intrinsic viscosity of from 4.0 to 8.0 dl/g and also having an ethylene content of from 5 to 50% by weight and a propylene-ethylene block copolymer having an intrinsic viscosity of not less than 2.0 dl/g but less than 4.0 dl/g and also having an ethylene content of more than 50% by weight but not more than 98% by weight are excluded. The publication also discloses that a thermoplastic resin composition with an extremely great ductility is obtained.
Moreover, JP-A-9-48831 discloses a propylene-ethylene block copolymer superior in impact resistance, stiffness and moldability which is obtained by multi-stage polymerization, the block copolymer comprising (a) from 60 to 96% by weight of homopolypropylene portion having a melt flow rate (MFR), as determined according to ASTM D-1238, of from 1 to 1000 g/10 min and satisfying the relationship ΔHm≧24.50+1.583logMFR wherein ΔHm is heat of fusion determined by differential scanning calorimetry, (b) from 2 to 38% by weight of a propylene-ethylene copolymer portion of lower ethylene having an ethylene content of from 20 to 50% by weight and an intrinsic viscosity of from 2 to 5 dl/g, and (c) from 2 to 38% by weight of propylene-ethylene copolymer portion of higher ethylene content having an ethylene content of from 50 to 90% by weight and an intrinsic viscosity of from 3 to 6 dl/g.
However, even in the propylene-ethylene block copolymer described in the above-cited published patent applications, further improvement in stiffness, hardness, moldability and also in balance between toughness and low-temperature impact resistance has been awaited.